package leetCode;

import java.util.List;

public class 合并两个有序链表 {

    /**
     * 递归实现
     *
     * @param l1
     * @param l2
     * @return
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        } else if (l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }

    public ListNode mergeTwoListsWhile(ListNode l1, ListNode l2) {

        ListNode prehead = new ListNode(-1);
        ListNode pre = prehead;

        while (l1 != null && l2 != null) {

            if (l1.val <= l2.val) {
                pre.next = l1;
                l1 = l1.next;
            } else {
                pre.next = l2;
                l2 = l2.next;
            }

            pre = pre.next;

        }
        // 合并后 l1 和 l2 最多只有一个还未被合并完，我们直接将链表末尾指向未合并完的链表即可
        if (l1 == null) {
            pre.next = l2;
        } else {
            pre.next = l1;
        }

        return prehead.next;
    }
}
